1. Field of the Invention
The present invention relates to an image forming apparatus such as an ink jet recording apparatus for which it is possible to determine a state of an ink droplet being discharged.
2. Description of the Related Art
An ink jet recording apparatus is known in which a pressure is caused to be generated in a liquid chamber, a liquid droplet is caused to discharge from a nozzle that is provided on the liquid chamber to a medium such as paper, and thus, printing is carried out on the medium. In particular, an ink jet recording apparatus of a so-called on-demand type is known in which many nozzles are arranged in an array, and, only when recording is carried out, a pressure is caused to be generated only in a liquid chamber corresponding to a desired one of the nozzles, and a liquid droplet is caused to be discharged by the nozzle. In the ink jet recording apparatus of the on-demand type, in a case where, for example, a standby time period in which recording is not carried out is long, it may become not possible to discharge ink droplets from the nozzles because a solvent of the ink existing in the nozzles evaporates and thus the viscosity of the ink increases, or because foreign materials or air bubbles exist in the ink. As a result, white lines, spots or such may occur in an image that the ink jet recording apparatus records and image quality may be thus degraded. Therefore, it may be necessary to check that liquid droplets can be properly discharged from the many nozzles arranged as mentioned above as a trial before actually carrying out printing, and carry out a recovery operation such as suctioning and ejecting the liquid from the nozzles which cannot discharge liquid droplets, and thus, it may be necessary that actual printing is carried out after almost all the nozzles become to be in states of being able to discharge liquid droplets.
As a method of thus determining whether ink droplets are discharged from nozzles, the following methods are known. A first method is such that liquid droplets to be discharged are caused to be electrically charged and an electrical charge flowing when the liquid droplets come into contact with a detection electrode is detected (see Patent Document 1 (Japanese Laid-Open Patent Application No. 2007-021782) and Patent Document 2 (Japanese Laid-Open Patent Application No. 11-170569)). A second method is such that liquid droplets to be discharged are caused to be electrically charged and an induced electric current that flows when the liquid droplets pass near a detection electrode is detected (see Patent Document 3 (Japanese Laid-Open Patent Application 2006-272634)). A third method is such that liquid droplets are caused to land between a pair of electrodes, and a change in resistance or dielectric constant between the electrodes is detected (see Patent Document 4 (Japanese Laid-Open Patent Application No. 2005-238682)).
Further, Patent Document 5 (Japanese Laid-Open Patent Application No. 2004-284314) discloses a liquid droplet discharge testing apparatus. The liquid droplet discharge testing apparatus is such that liquid droplets for testing are electrified and are caused to be discharged from nozzle holes, and the liquid droplets are deflected by means of a deflection electric field and are caused to land on a deflection electric field generating electrode and liquid droplet detecting member that is provided on a nozzle surface. As a result, an electric current flows through the deflection electric field generating electrode and liquid droplet detecting member, and it is determined as a result of the electric current being sensed that the liquid droplets have been discharged. This configuration requires a deflection electric field generating electrode per nozzle row on the nozzle surface, cleaning on the nozzle surface may thus become difficult, and also, it may be difficult to clean the deflection electric field generating electrode and liquid droplet detecting member that has become dirty by the liquid droplets having landed. Further, at a time of the testing, it is necessary that the liquid droplets for testing are discharged in such a condition that the speeds of the liquid droplets are made to be lower or the weights of the liquid droplets are made to be smaller so that flying directions of the liquid droplets can be greatly bent (deflected). Thus, complicated control may be required.